The present invention relates to microwave components used in radio and telephone communications. Specifically, an integrated circuit transformer is disclosed which can be used as a multi-winding balun transformer in radio frequency applications.
SiGe integrated technology has been used to manufacture integrated circuits to implement RF receivers as well as RF transmitters in wireless personal communications systems. Radio frequency signal circuits typically require inductances or transformers having multiple windings to perform standard radio frequency circuit functions. Wireless telephones, for instance, require receivers which have tuned circuit inductors and transformers in the first RF section, and have mixer circuits which use transformers for converting the received microwave signals into intermediate frequency signals. Compared to other passive circuit components, the integration of inductances and transformer elements for performing these functions is considerably more difficult in VLSI technology. Specifically, whereas micro-strip components may be used to create inductances and transformer windings, attempts to integrate them in VLSI technology has been limited by the high losses that substrate dielectrics present to a micro-strip component.
Attempts have been made to implement a transformer windings in multi-layer metallization surfaces separated by a silicon oxide insulation layer. These attempts, however, have been unsuccessful in controlling the high parasitic capacitance which occurs between windings on different metallization layers of a VLSI circuit. The high parasitic capacitance is especially troublesome when using a transformer in a differential mode application since the parasitic capacitance degrades common mode rejection which is desired in circuits which are intended to operate in a differential mode.
The present invention is directed to improvements in transformer designs for such VLSI applications.